This invention relates to processes and compositions for the generation of acid.
Many processes are known using a medium which, when irradiated with electromagnetic radiation, generates an acid. This acid is then used to cause a change in the properties of the medium, so that exposed and unexposed portions of the medium differ in their properties. For example, besides the processes described in the two aforementioned copending applications, many photoresist compositions are of this type; the acid produced upon exposure to (typically) ultra-violet radiation changes the solubility of the photoresist composition in the solution used to develop the photoresist.
In most prior art acid-generating photoresist processes, the sensitivity of the medium to the exposing radiation is not of major concern, since exposure is normally effected using powerful ultra-violet sources and long exposure times can be tolerated. However, the processes described in the two aforementioned applications are primarily intended for imaging with near infra-red radiation using radiation from semiconductor diode lasers; such diode lasers have the advantage of being much less expensive than ultra-violet lasers, while being well adapted for the production of high resolution images and for digital imaging processes (i.e., for producing hard copies of images stored on computers in digital form). The cost per unit intensity for a high-resolution addressable source is higher for ultra-violet radiation than for infra-red radiation. However, the imaging speed of such processes using infra-red radiation is presently limited by the sensitivity of the medium, and accordingly, there is a need to develop infra-red sensitive imaging media with improved sensitivity.
The sensitivity of such photosensitive compositions can be increased if the photosensitive molecule catalyzes a secondary reaction which is not radiation-dependent and effects conversion of several molecules for each quantum absorbed. For example, photoresist systems are known in which the primary photochemical reaction produces an acid, and this acid is employed to eliminate acid-labile groups in a secondary, radiation-independent reaction. See, for example, U.S. Pat. Nos. 3,932,514 and 3,915,706; Reichmanis et al., Chemical Amplification Mechanism for Microlithography, Chem. Mater., 3(3), 394 (1991) and Berry et al., Chemically Amplified Resists for I-line and G-line Applications, SPIE, 1262, 575 (1990). Also, U.S. Pat. No. 5,084,371 describes a radiation-sensitive mixture which contains a water-insoluble binder which comprises a mixture of phenolic and novolak polymers soluble or dispersible in aqueous alkali, and an organic compound whose solubility in alkaline developer is increased by acid, and which also contains at least one acid-cleavable group, and in addition a further group which produces a strong acid upon exposure to radiation. Similarly, in the aforementioned application Ser. No. 07/965,161, a secondary acid generator (when used) " amplifies" the acid produced by the iodonium salt or other superacid precursor, resulting in several molecules of acid being produced for each molecule of superacid originally produced by decomposition of the iodonium salt. However, despite the increase in sensitivity achieved by such acid amplification, the contrast, and hence the quality, of the resultant image is still governed by the original photochemical acid generation step. Accordingly, it is desirable to secure as high a quantum yield as possible during the photochemical acid generation step.
The present inventors have carried out extensive investigations regarding the factors which significantly affect the quantum yield of infra-red sensitive acid-generating reactions, and have determined that, when the acid precursor employed is a diaryl iodonium salt, certain squarylium dyes are effective in acid generation. The binder used in the imaging medium also has an important influence on the quantum yield.